The art of lithographic printing is based on the immiscibility of oil and water. Ink receptive areas are generated on the surface of a hydrophilic surface. When the surface is moistened with water and then ink is applied, the hydrophilic background areas retain the water and repel the ink and the ink receptive areas accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Lithographic printing plates typically comprise a radiation-sensitive coating applied to a support. If after exposure to radiation, the exposed portions of the coating become soluble and are removed in the developing process, the plate is called as a positive-working printing plate. Conversely, if exposed portion of the plate become insoluble in the developer and the unexposed portions are removed by the developing process, the plate is called a negative-working plate. In each instance the portions of the radiation-sensitive layer (i.e., the image areas) that remain are ink-receptive.
Infrared-sensitive imaging elements for the preparation of positive-working lithographic printing plates have been disclosed comprising a substrate, an aqueous alkali soluble underlayer, and a radiation-sensitive top layer. On exposure, the exposed areas of the top layer become soluble or permeable in aqueous alkali so that the developer can penetrate the top layer and remove the underlayer, exposing the underlying substrate. Systems have been produced in which a developer insoluble top layer is coated over a developer soluble underlayer. Following exposure both layers are removed by the developer in the exposed region, revealing the hydrophilic surface of the underlying substrate.
In use, a lithographic printing member comes in contact with fountain solution. In addition, the printing member is often subjected to aggressive blanket washes, such as a "UV wash" to remove ultraviolet curable inks. However, many of these systems have limited resistance to either fountain solution and/or aggressive blanket washes. Thus, a need exists for an improved imageable element, useful as a lithographic printing member, that does not suffer from these disadvantages.